In some prior art inverter fed induction motors, for example, used with some washing machines and similar motor applications, motor speed reversal is a normal occurrence. To avoid over current and voltage conditions during speed reversal, those skilled in the art have attempted to avoid over current and voltage conditions through different techniques.
For example, in one prior art technique, it is possible to disable all pulses to the inverter, and let the motor coast with its inertia to zero speed, and then restart in a reverse direction. A disadvantage to this approach is that coasting takes more time to stop. It is also possible to use vector control. Although this has been found to be an adequate approach for controlling voltage and current during acceleration and deceleration of induction motors, it is expensive. Vector control requires a powerful Digital Signal Processor (DSP) and various sensors, and thus, typically has not always been cost effective. Another approach over designs the power inverter stage to accommodate the voltage and current surges that are expected in end-use applications. This is expensive and usually requires a larger circuit footprint, since the entire circuit is able to handle such voltage and current surges, no matter how rare. In other cases, during motor reversal, detection of over voltage and over current conditions has resulted in removing gate pulses to let the motor coast to zero speed and then restart. This has not always been found adequate.